Abstract

Conventional engineering hydrologic analysis of the 1976 Colorado centennial Big Thompson flood indicates that the flood event, while catastrophic, was not extremely unusual. The geologic evidence in the area immediately below the area of maximum runoff, however, indicates that a flood of this magnitude had not occurred in the main channel for several thousand years. Alluvial-fan deposits extensively eroded by the flood waters were dated by observations of hue oxidation, coring of trees, and 14C dating of buried charcoal. Some eroded deposits were as old as 7,000 to 10,000 14C yr. Where usable, geologic evaluations of extreme flood events may yield more accurate estimates of the risk of large, rare floods than conventional methods.

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